Promising interlayer sensitization strategy for the construction of high-performance blue hyperfluorescence OLEDs

Multi-resonance thermally activated delayed fluorescence (MR-TADF) materials are promising candidates for organic light-emitting diodes (OLEDs) with narrow electroluminescence (EL) spectra. Current researches focus on fabricating hyperfluorescence OLEDs to improve EL efficiencies of MR-TADF emitters by co-doping them with TADF sensitizers in a single host layer. However, in many cases, the polarity of the single host could be not suitable for both blue MR-TADF emitters and blue TADF sensitizers, resulting in broadened EL spectra in high-polar hosts or decreased EL efficiencies in low-polar hosts. Herein, we wish to report an efficient sensitization strategy for blue MR-TADF emitters by constructing an interlayer-sensitizing configuration, in which the blue TADF sensitizers and blue MR-TADF emitters are separated into two closely aligned host layers with high polarity and low polarity, respectively. Based on this strategy, efficient blue hyperfluorescence OLEDs are realized and verified by employing various TADF sensitizers and different MR-TADF emitters, furnishing outstanding external quantum efficiencies of up to 38.8% and narrow EL spectra. These results validate the feasibility and universality of this interlayer sensitization strategy, which provides an effective alternative to high-performance blue hyperfluorescence OLEDs.


Introduction
High-quality organic light-emitting diode (OLED) displays require high efficiency, high brightness, high stability, and high color purity simultaneously [1][2][3][4] .With the light-emitting materials of OLEDs developing to the third generation of purely organic thermally activated delayed fluorescence (TADF) materials, the efficiency, brightness, and stability have been greatly improved, and some parameters have fulfilled the requirements for the commercialization of OLEDs [5][6][7][8] .However, due to the strong intramolecular charge transfer (CT) effect, normal TADF materials featuring a twisted donor-acceptor (D-A) configuration exhibit broad emission spectra, which makes it difficult for them to acquire high color purity [9][10][11][12][13][14] .
Thanks to the advent of multi-resonance TADF (MR-TADF) materials, it becomes realizable readily to fabricate high-performance OLEDs with superb color purity [15][16][17] .In MR-TADF molecules, the electron density populations of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) are alternately distributed on the atoms due to the complementary resonance effect.Such kind of frontier molecular orbital distributions in a rigid molecular framework can minimize vibronic coupling and vibrational relaxation of the excited states, leading to a series of extraordinary photophysical properties such as high photoluminescence quantum yields (PLQY or Φ PL ), fast radiative decay, and narrow full-width at half-maximum (FWHM) of emission spectra [18][19][20][21] .Therefore, MR-TADF materials are promising candidates to improve the color quality of OLEDs.
Although MR-TADF molecules show TADF characteristics, they usually have relatively large energy splitting (ΔE ST ) between the lowest excited singlet (S 1 ) state and the lowest excited triplet state (T 1 ), which leads to slow reverse intersystem crossing (RISC) and thus long delayed fluorescence (DF) lifetime 22,23 .Consequently, many MR-TADF molecules suffer from serious tripletinvolved quenching processes, which undermine electroluminescence (EL) efficiencies and cause large efficiency roll-offs at high luminance.Many current researches focus on addressing this issue by fabricating hyperfluorescence devices, in which conventional TADF sensitizers with short DF lifetimes are incorporated to sensitize MR-TADF molecules [24][25][26][27][28][29] .So far, the EL performances of many MR-TADF molecules have been successfully improved by this approach.But for blue MR-TADF molecules, the current sensitization configuration of co-doping MR-TADF emitters with TADF sensitizers in the same wide-energy-gap host layer may not be the best option.The reason is that the polarity of a single host could be not suitable for both blue TADF sensitizer and blue MR-TADF emitter at the same time.Generally, the weak D-A interaction of blue TADF sensitizers in lowpolar hosts will have a negative impact on the TADF property and the RISC process, and the increasing tripletinvolved quenching process in blue TADF sensitizers will decrease EL efficiencies.Therefore, most blue TADF sensitizers prefer high-polar hosts to improve the TADF property.However, the high-polar hosts will lead to an enhanced CT effect of the blue MR-TADF emitters, which will broaden the EL spectra and decrease the PLQYs of the blue MR-TADF emitters.Therefore, blue MR-TADF emitters require low-polar hosts to prevent the broadened EL spectra and decreased PLQY.
In this work, we wish to report an effective interlayersensitizing configuration for blue MR-TADF emitters, in which TADF sensitizer and MR-TADF emitter are separated into two adjacent emitting layers (EMLs) with different hosts, and MR-TADF emitters can be sensitized by TADF sensitizers via long-range Förster energy transfer (FET), as displayed in Fig. 1.Based on this interlayer sensitization strategy, high-performance hyperfluorescence OLEDs using different blue MR-TADF emitters are prepared, furnishing strong blue light with narrow EL spectra and outstanding external quantum efficiencies (EQEs) of up to 38.8%, which are apparently improved in comparison with those of unsensitized devices.These results demonstrate that this interlayer sensitization strategy is applicable for the fabrication of blue hyperfluorescence OLEDs with high EL efficiency and high color purity simultaneously.

Results
To validate the feasibility of this interlayer sensitization strategy, efficient energy transfer from TADF sensitizers to MR-TADF emitters should be ensured 30 .According to the influencing factors in the FET radius (R 0 ) formula given in supplementary information, the Φ PL s of the TADF sensitizers and the spectral overlaps between the PL spectra of the TADF sensitizers and the absorption spectra of the MR- with high EL performance is chosen as the emitter, and three efficient blue TADF materials (TBCz-XT, DMAC-DPS, and PPCzTrz) [32][33][34] with totally different molecular structures are selected as the sensitizers.The molecular structures of these materials are displayed in Fig. 2a.As displayed in Table S2 and Fig. S1, three chosen blue TADF sensitizers all have Φ PL s of over 90%, and the PL spectra of these TADF sensitizers have large overlaps with the absorption spectra of the MR-TADF emitters.
The hyperfluorescence devices (IS1, IS2, and IS3) are fabricated with a configuration of ITO/HATCN (5 nm)/ TAPC (50 nm)/TCTA (5 nm)/mCBP (5 nm)/1 wt% BNCz-pTPA: mCBP (10 nm)/sensitizing layer (2 nm)/PPF (5 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (120 nm) (Fig. 2b).Thereinto, the sensitizing layers are configured  carbazol-9yl)biphenyl (mCBP) is used as the host for MR-TADF emitter BNCz-pTPA as well as the electronblocking layer, bis-(diphenylphosphoryl)-dibenzo[b,d]furan-synonym (PPF) is used as the host for TADF sensitizers (TBCz-XT, DMAC-DPS and PPCzTrz) as well as the hole-blocking layer, 1,3,5-tri(m-pyrid-3-yl-phenyl) benzene (TmPyPB) is the electron-transporting layer, and LiF/Al serves as cathode.The thickness of the sensitizing layer should be kept within the R 0 of the blue TADF sensitizers, commonly in the range of 2-4 nm 35,36 , otherwise parts of energy cannot be transferred from TADF sensitizers to MR-TADF emitters, leading to the appearance of intrinsic emissions of TADF sensitizers.Moreover, to prevent concentration quenching, the doping concentration of MR-TADF emitter is usually controlled at a very low level, which will also cause incomplete energy transfer of the TADF sensitizers.Consequently, in order to cover more MR-TADF molecules within the energy transfer range of the TADF sensitizers, the sensitizing layer should be designed as thin as possible.However, sufficient electrogenerated excitons are essential for high EL performances, so the thickness of the sensitizing layer cannot be too small.Here, considering the R 0 of these blue TADF sensitizers (Table S2) and the potential incomplete energy transfer caused by doping concentration difference, the thickness of the sensitizing layer is set as 2 nm in these devices.
Since the blue TADF sensitizers prefer high-polar hosts to low-polar ones, the co-phase sensitized hyperfluorescence devices (S4-S6) with the same configuration as devices S1-S3 except for using the high-polar host PPF are also prepared (Fig. 2d).Devices S4-S6 show peak EQEs of 29.5%, 31.8% and 33.2% (Fig. S8, Table 1), respectively, slightly higher than those of devices S1-S3, confirming the high-polar PPF host indeed has positive effect on improving EL efficiencies.However, these EQEs are still obviously lower than those of the interlayersensitized hyperfluorescence devices IS1-IS3.In addition, devices S4-S6 exhibit EL spectra with FWHMs of 32 nm, broader than those of the unsensitized devices (28 nm) as well as the interlayer-sensitized devices (30 nm).The lower EL efficiencies and broader EL spectra of devices S4-S6 should be attributed to the decreased PLQY and strengthened CT effect of the MR-TADF emitter caused by the high-polar host PPF.
To confirm the discussions above, the PLQY and PL spectra of the doped films of 1 wt% BNCz-pTPA: PPF and 1 wt% BNCz-pTPA: mCBP are measured.The PLQY of the film of 1 wt% BNCz-pTPA: PPF is 85%, lower than that of the film of 1 wt% BNCz-pTPA: mCBP (95%), which is responsible for the lower EQEs of the devices with high-polar PPF host.Meanwhile, as shown in Fig. S9, the PL spectrum of the film of 1 wt% BNCz-pTPA: PPF has a FWHM of 29 nm, broader than that of the film of 1 wt% BNCz-pTPA: mCBP (26 nm), evidencing that highpolar host will broaden the EL spectrum of the MR-TADF emitter.
It has been reported that a similar device configuration with two separated adjacent phases could reduce shortrange Dexter energy transfer (DET), which is beneficial for higher EL efficiencies 39 .Differently, the interlayersensitizing device configuration in this work is specifically proposed for optimizing blue MR-TADF emitters and the main reason for the improved device efficiencies is that the blue MR-TADF emitter and blue TADF sensitizer are both doped in wide-energy-gap hosts with suitable polarity.To exclude the influence of DET, the interlayersensitized hyperfluorescence devices (S7-S9) using the same host mCBP are fabricated for comparison.The structures of devices S7-S9 are the same as those of devices IS1-IS3 except that the sensitizing layers are configured as 20 wt% TBCz-XT: mCBP, 20 wt% DMAC-DPS: mCBP and 10 wt% PPCzTrz: mCBP for devices S7, S8 and S9 (Fig. 2e), respectively.The peak EQEs of devices S7, S8 and S9 are 23.4%, 23.8% and 25.8% (Fig. S10, Table 1), much lower than those of interlayer-sensitized hyperfluorescence devices IS1-IS3, and even lower than those of co-phase sensitized hyperfluorescence devices S1-S3.These results suggest that the efficiency improvement is attributed to the suitable polarity of the wideenergy-gap hosts instead of the decrease of DET.Besides, it can be seen from Fig. S10a that devices S7-S9 all show obvious emissions from TADF sensitizers.The emissions of TADF sensitizers (TBCz-XT and PPCzTrz) in devices S7 and S9 are peaked at about 450 nm, corresponding to the EL peaks of devices SB1 and SB5.The EL peak of the TADF sensitizer DMAC-DPS in mCBP host (device SB3) is located at about 470 nm, close to the EL peak of MR-TADF emitter BNCz-pTPA (about 490 nm).So, the emission spectrum of TADF sensitizer DMAC-DPS is mostly covered by the emission spectrum of the MR-TADF emitter, which makes the right half of the EL spectrum of device S8 obviously broader than those of devices S7 and S9.
To explain the obvious emissions of TADF sensitizers generated in devices S7-S9, the carrier mobility of different doped layers in devices S7-S9 (1 wt% BNCz-pTPA: mCBP, 20 wt% TBCz-XT: mCBP, 20 wt% DMAC-DPS: mCBP and 10 wt% PPCzTrz: mCBP) are investigated via space charge limited current (SCLC) method 40 .Hole-only devices with the configuration of ITO/MoO 3 (10 nm)/ doping layer (80 nm)/MoO 3 (10 nm)/Al (120 nm) and electron-only devices with the configuration of ITO/ TmPyPB (10 nm)/doping layer (80 nm)/TmPyPB (10 nm)/ LiF (1 nm)/Al (120 nm), respectively, are fabricated, in which MoO 3 and TmPyPB functioned as buffer layers to shield off electrons and holes between the doped layers and the electrodes, respectively.Owing to the much thicker doped layers (80 nm), the influence of the buffer layers can be excluded.The electric field-dependent mobilities of the different doped layers are displayed in Figs.S11, S12.It can be seen that all the hole mobilities (10 −3 -10 −2 cm 2 V −1 s −1 ) in devices S7-S9 are several orders of magnitude larger than the electron mobilities (10 −6 -10 −5 cm 2 V −1 s −1 ).In consequence, most excitons will recombine at the interface between the sensitizing layer and the hole-blocking layer, as shown in Fig. 5a.The thickness of the sensitizing layer is set as 2 nm, so the energy of TADF sensitizers can only be transferred to MR-TADF molecules within the thickness range of (R 0 -2) nm through FET, and considering the low doping concentration of MR-TADF emitter, the excitons generated on the TADF sensitizers will be more than the MR-TADF molecules that can accept energy.Therefore, parts of the excitons cannot be transferred to MR-TADF molecules and thus will undergo direct radiative decay on TADF sensitizers.For comparison, the carrier mobilities of the different sensitizing layers in devices IS1-IS3 (20 wt% TBCz-XT: PPF, 20 wt% DMAC-DPS: PPF and 10 wt% PPCzTrz: PPF) are also investigated and displayed in Fig. S13.Different from devices S7-S9, the hole mobilities (10 −5 -10 −4 cm 2 V −1 s −1 ) of the sensitizing layers in devices IS1-IS3 are close to their electron mobilities (10 −6 -10 −4 cm 2 V −1 s −1 ).Therefore, after holes quickly passing through the EML (1 wt% BNCz-pTPA: mCBP) with high hole mobility (10 −3 -10 −2 cm 2 V −1 s −1 ), the holes and electrons will move with similar mobilities in the sensitizing layer and recombine into excitons, and at this time the thickness range for the MR-TADF molecules to accept energy is larger than (R 0 -2) nm, as displayed in Fig. 5b.Consequently, compared with devices S7-S9, the emissions of TADF sensitizers in devices IS1-IS3 are greatly reduced, which further proves the importance of the appropriate host combination for TADF sensitizers and MR-TADF emitters in the interlayer-sensitized hyperfluorescence devices.
The above results and analyses manifest that the interlayer-sensitizing system is advantageous to optimize some blue MR-TADF emitters compared with the cophase sensitizing system using a single wide-energy-gap host.Then, the universality of this strategy is examined by introducing it to other blue MR-TADF emitters of v-DABNA and t-DABNA 41,42 , and above three TADF materials are still employed as sensitizers.The molecular structures of v-DABNA and t-DABNA are displayed in Fig. 2a, and the absorption spectra of v-DABNA and t-DABNA can well-overlapped with the emission spectra of these three TADF sensitizers (Fig. S1).
There are weak emissions from the TADF sensitizers observed for the spectra of the interlayer-sensitized hyperfluorescence devices.The primary reason is the distance gradient between the TADF sensitizer and the MR-TADF emitter induced by the interlayer configuration.As the distance between the exciton on TADF sensitizer and MR-TADF molecule increases, the FET efficiency will decrease.Given the R 0 s of the TADF sensitizers are relatively small, ranging from 3.3-4.5 nm, parts of excitons on TADF sensitizers are unable to transfer energy because of the low FET efficiency, leading to the emission of the TADF sensitizers, especially for the excitons recombined in proximity to the interface of sensitizing layer and hole-blocking layer (Fig. S10a).To further monitor the spectral changes with increased voltages, the EL spectra of devices IS1-IS9 at different voltages are tested.As shown in Fig. S15-S17, the emission of the TADF sensitizers increases slightly by rising voltage, which may result from the increased exciton concentration on TADF sensitizers but low FET efficiency at high current density, and the elongated distance between the TADF sensitizer and the MR-TADF emitter induced by the degradation at the EML/sensitizing layer interface.

Discussion
In summary, to improve the EL performances of MR-TADF emitters, hyperfluorescence OLEDs were proposed, which are generally designed by co-doping MR-TADF emitters with conventional TADF sensitizers into a single host layer.However, the different requirements for host polarity of MR-TADF emitters and TADF sensitizers often lead to difficulties in acquiring high EQEs and narrow EL spectra simultaneously, particularly for blue hyperfluorescence OLEDs.To address this issue, herein, an interlayer sensitization strategy is reported for blue MR-TADF emitters.The interlayer-sensitizing system is designed by separating MR-TADF emitters and TADF sensitizers into two adjacent host layers with high polarity and low polarity, respectively.Blue MR-TADF emitters can be sensitized via the FET mechanism to realize high EQEs and maintain narrow EL spectra in the low-polar hosts.Based on the interlayer sensitization strategy, the state-ofthe-art blue hyperfluorescence OLEDs with high color quality and outstanding EQEs of up to 38.8% are realized.This strategy is also effective for different blue MR-TADF emitters and different blue TADF sensitizers, demonstrating the feasibility and universality in the construction of high-performance blue hyperfluorescence OLEDs.

Materials and instruments
The compounds HATCN, TAPC, TCTA, mCBP, PPF, TmPyPB, DMAC-DPS, PPCzTrz, v-DABNA, and t-DABNA were purchased from commercial sources.BNCz-pTPA and TBCz-XT were synthesized according to previously reported methods.UV-vis absorption spectra were measured on a Shimadzu UV-2600 spectrophotometer.PL spectra were recorded on a Horiba Fluoromax-4 spectrofluorometer.PL quantum yields were measured using a Hamamatsu absolute PL quantum yield spectrometer C11347 Quantaurus_QY.Transient PL decay spectra were measured using Edinburgh Instruments FLS1000 spectrometer.

Device fabrication and characterization
The glass substrates precoated with a 90-nm layer of ITO with a sheet resistance of 15~20 Ω per square were successively cleaned in ultrasonic bath of acetone, isopropanol, detergent, and deionized water, respectively, taking 10 minutes for each step.Then, the substrates were totally dried in a 70 °C oven.Before the fabrication processes, in order to improve the hole injection ability of ITO, the substrates were treated by O 2 plasma for 10 minutes.The vacuum-deposited OLEDs were fabricated under a pressure of <5 × 10 -4 Pa in the Suzhou Fangsheng OMV-FS380 or OMV-FS450 vacuum deposition system.Organic materials, LiF and Al were deposited at rates of 1~2 A s -1 , 0.1 A s -1 , and 5 A s -1 , respectively.The effective emitting area of the devices was 9 mm 2 , determined by the overlap between anode and cathode.The luminance-voltage-current density and EQE were characterized with a dual-channel Keithley 2614B source meter and a PIN-25D silicon photodiode.The EL spectra were obtained via an Ocean Optics USB 2000+ spectrometer with a Keithley 2614B source meter and a PhotoResearch PR670 spectroradiometer with a Keithley 2400 Source Meter.All the characterizations were conducted at room temperature in ambient conditions without any encapsulation, as soon as the devices were fabricated.

Fig. 2
Fig. 2 Materials information and device configurations of OLEDs.a Molecular structures of MR-TADF emitters and TADF sensitizers employed in this work.Device configurations of b devices IS1-IS9, c devices S1-S3, d devices S4-S6, and e devices S7-S9

Fig. 3
Fig. 3 Device data and EL performance of devices IS1-IS3.a EL spectra at 5 V, b current efficiency and power efficiency versus luminance curves, c external quantum efficiency versus luminance curves, and d luminance and current density versus voltage curves of devices IS1-IS3

Fig. 4
Fig. 4 Mechanism study of the sensitizing process.Transient PL decay curves of a doped films I and IV, b doped films II and V, and c doped films III and VI, detected at 450 nm

Fig. 5
Fig. 5 The exciton behaviors of the interlayer-sensitizing systems.a The same hosts for TADF sensitizers and MR-TADF emitters.b The different hosts for TADF sensitizers and MR-TADF emitters

Fig. 6
Fig. 6 Device data and EL performance of devices IS4-IS9.a EL spectra at 5 V, b external quantum efficiency versus luminance curves, and c luminance and current density versus voltage curves of devices IS4-IS6.d EL spectra at 5 V, e external quantum efficiency versus luminance curves, and f luminance and current density versus voltage curves of devices IS7-IS9

Table 1
EL performances of hyperfluorescence OLEDs